Over the past several decades, there has been a surge of interest in the synthesis and properties of conducting polymers. These polymers are typically synthesized by one of three general methods, including chemical (Allcock and Dodge, 1992 Chem Mater 4: 780), electrochemical (Diaz and Bargon in Handbook of Conducting Polymers, Vol. 1, Chapter 3, Skotheim, ed., Marcel Dekker: New York, 1986), and plasma polymerization (Wang et al., 2004, Thin Solid Films 446: 205). Numerous applications using conducting polymers have been proposed, ranging from molecular electronics to anti-corrosive agents. Despite the promise of these new materials, relatively few viable technologies have emerged from proof-of-concept laboratory studies. One of the biggest impediments to the successful implementation of these polymers has been their lack of processability. That is, these polymers cannot be melted or extruded nor are they soluble in many solvents. Therefore, they are not easily processed, for example, molded or painted. Several elegant approaches have been developed over the years to impart processability. For example, the addition of bulky side chains along the backbone can disrupt π-π interactions resulting in soluble conducting polymers. However, this approach invariably leads to lower conductivities (Jang et al., 2004, Macromolecules 37: 4351) due to reduced π-orbital overlap along the backbone (Scherman and Grubbs, 2001, Polymeric Materials Science and Engineering 84: 603). Alternative approaches use emulsions or suspensions that can be processed; however they typically retain the original microstructure present in solution. Recently it was demonstrated that homogeneous polymer structures can be created by flash welding films consisting of nanoparticles of conducting polymers; however more work is required to determine of how this thermal processing impacts the electronic properties of the polymer since conductivities obtained decreased by an order of magnitude.
In this work we explore an alternative strategy involving the use of metastable mixtures of monomer and oxidant that enable processability followed by in situ polymerization initiated by solvent evaporation. This approach was originally demonstrated with pyrrole/phosphomolybdic acid mixtures that were used to produce will-behaved polypyrrole films (Freund et al., 1995, Inorganica Chimica Acta 240: 447) that could be deposited on a variety of substrates enabling previously un-reported applications including composite polymer-based sensing arrays (Freund and Lewis, 1995, PNAS 92: 2652) and hybrid electronic devices (Lonergan, 1997, Science 278: 2103). The proposed mechanism responsible for this process involves the formation of a metastable mixture of oxidant and monomer by selecting an oxidant whose formal potential is close to, but lower than, the oxidation potential of the monomer. This insures that the concentration of oxidized monomer (a radical cation) is relatively low, thereby resulting in a relatively slow polymerization rate (a radical coupling reaction). While the solutions are metastable under dilute conditions, when concentrated (upon solvent evaporation) the rate-limiting radical coupling reaction becomes significantly faster, resulting in a rapid increase in the concentration of n-mers that in turn have lower oxidation potentials with their increased conjugation length (Diaz et al., 2000, J Am Chem Soc 122: 12385). The increased concentration of radical cations, resulting from the more favourable thermodynamics, causes a further increase in the polymerization rate as the reaction cascades.
This synthetic strategy should be general and applicable to any polymer system involving a similar redox-driven polymerization. Theoretically, all that is required is the proper balance of relative redox potentials of the monomer and oxidant as well as concentration and solvent evaporation rate. Polythiophene is a more stable conducting polymer that has proven to be a useful material in a wide range of technologies, including charge dissipating films (Heywang and Jona, 1994, Electrochimica Acta 39: 1345), light-emitting diodes (Frechet et al., 2000, J Am Chem Soc 122: 12385), electrochromic devices (Reynolds et al., 2000, Chem Mater 12: 1563), and organic vapour sensors (Briglin et al., 2000, Anal Chem 72: 3181). Since the oxidation potential for thiophene is higher than pyrrole (2.07 and 1.30 V vs. SCE, respectively) the synthetic approach requires altering the oxidation potential of either the monomer or phosphomolybdic acid. In this case it is straightforward to manipulate the oxidation potential of the monomer by utilizing either bithiophene (1.31 V) or terthiophene (1.05 V), which have redox potentials near of that of pyrrole (1.30 V). The formal potential of phosphomolybdic acid is 0.36 V. In addition, solvent and concentration must be taken into to insure the formation of a metastable solution.